The invention relates to optical switching arrangements and more particularly to arrangements of optical switching units for selectively manipulating optical signals from input and add ports to corresponding output and drop ports in optical add-drop multiplexers.
While signals within telecommunications and data communications networks have traditionally been exchanged by transmitting electrical signals via electrically conductive lines, an alternative mode of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals are sometimes satisfied by converting the optical signals to electrical signals at the inputs of a switching network, and then reconverting the electrical signals to optical signals at the outputs of the switching network.
Recently, reliable optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from any one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. An isolated switching unit 10 is shown in FIG. 1. The switching unit includes planar waveguides that are formed by layers on a substrate. The waveguide layers include a lower cladding layer 14, an optical core 16, and an upper cladding layer, not shown. The optical core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers are formed of a material having a refractive index lower than that of the core material, so that optical signals are guided along the core.
The layer of core materials 16 is patterned into waveguide segments that define a first input waveguide 20 and a first output waveguide 26 of a first optical path and define a second input waveguide 24 with a second output waveguide 22 of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap is formed by etching a trench 28 through the core material, the upper cladding layer, and at least a portion of the lower cladding layer 14. The first input waveguide 20 and the second output waveguide 22 intersect a sidewall of the trench 28 at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the junction 30 of the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching fluid resides within the junction 30 between the aligned input and output waveguides 20 and 26. The trench 28 is positioned with respect to the four waveguides such that one sidewall of the trench passes through or is slightly offset from the intersection of the axes of the waveguides.
The above-identified patent of Fouquet et al. describes a number of alternative approaches to switching the optical switching unit 10 between a transmissive state and a reflective state. One approach is illustrated in FIG. 1. The switching unit 10 includes a microheater 38 that controls formation of a bubble within the fluid-containing trench. While not shown in the embodiment of FIG. 1, the waveguides of a switching matrix are typically formed on a waveguide substrate and the heaters and heater control circuitry are integrated onto a heater substrate that is bonded to the waveguide substrate. The fluid within the trench has a refractive index that is close to the refractive index of the core material 16 of the four waveguides 20-26. Fluid fill-holes 34 and 36 may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching unit, the heater 38 is brought to a temperature sufficiently high to form a bubble in the index-matching fluid. Once formed, the bubble can be maintained in position by maintaining power to the heater. In FIG. 1, the bubble is positioned at the junction 30 of the four waveguides. Consequently, an input signal along the waveguide 20 will encounter a refractive index mismatch upon reaching the sidewall of the trench 28. This places the switching unit in a reflective state, causing the optical signal along the waveguide 20 to be redirected to the second output waveguide 22. However, even in the reflective state, the second input waveguide 24 is not in communication with the first output waveguide 26.
If the heater 38 at junction 30 is deactivated, the bubble will quickly condense and disappear. This allows index-matching fluid to fill the junction 30 for the waveguides 20-26. Since input signals will not encounter a significant change in refractive index at the interfaces of the input waveguides 20 and 24 with the trench 28, the switching unit 10 is then in the transmissive state. In the transmissive state, the optical signals along the first input waveguide 20 will propagate through the trench to the first output waveguide 26, while optical signals that are introduced via the second input waveguide 24 will propagate through the trench to the second output waveguide 22.
Matrices of the switching elements 10 may be used to form complex switching arrangements. A switching matrix may have any number of input ports (N) and any number of output ports (M), with each port being connected to an optical fiber. The fluid-controlled switching units allow the arrangement to be a strictly xe2x80x9cnon-blockingxe2x80x9d matrix, since any free input fiber may be optically coupled to any free output fiber without rearrangement of the existing connections.
Another type of switching matrix is an add/drop multiplexer that includes add ports and drop ports in addition to the input and output ports. Such multiplexers are utilized in telecommunications applications in which signals are passed through a series of nodes, with each node being able to introduce additional signals and being able to extract those signals that identify that node as a target. For example, each node may be a switching facility of a long distance carrier that supports calls to and from a number of cities. Calls that originate in a city are introduced using add ports within the switching facility of that city. On the other hand, data and voice information for calls directed to a telephone supported by that switching facility are extracted via drop ports. A known switch 40 that can be used as a rearrangeable add/drop switch is shown in FIG. 2. The arrangement includes a 4xc3x974 matrix of optical switching units for selectively coupling any one of four input ports 42, 44, 46 and 48 to any one of four output ports 52, 54, 56 and 58. In FIG. 2, each of the switching units that is in a reflective state is shown as having a bubble at the area at the intersection of input and output waveguides to that switching unit. Thus, switching units 62, 64, 66 and 68 are each in a reflective state. The remaining twelve switching units are in a transmissive state, since there are no bubbles present at the intersections of the input and output waveguides to those switching units.
Optical fibers are connected to each of the input ports 42-48 and each of the output ports 52-58. An optical signal that is introduced at the input port 42 will be reflected at the switching unit 62 and will be output via the output port of 54. Similarly, an optical signal from the input port 44 will reflect at the switching unit 64 for output at the port 56. An optical signal from the input port 46 reflects at the switching unit 66 for output via the port 58. Finally, an optical signal on port 48 is reflected to output port 52 by the switching unit 68. By selectively manipulating the bubbles within the various trenches, any one of the input ports can be connected to any one of the output ports.
The switch 40 includes four add ports 72, 74, 76 and 78. Each add port is uniquely associated with one of the output ports 52-58, since an optical signal that is introduced at one of the add ports can be directed only to its aligned output port. Thus, an optical signal on add port 72 can be directed to the output port 52 by changing the switching unit 68 to the transmissive state. This change to the transmissive state places the input port 48 in optical communication with a drop port 88. The drop port 88 is uniquely associated with the input port 48, since the drop port cannot be optically coupled to any other input or add port. Similarly, each one of the three other drop ports 82, 84 and 86 is uniquely associated with one of the input ports 42, 44 and 46, respectively, with which the drop port is linearly aligned.
A concern with the optical switch 40 of FIG. 2 is that there is a relatively large cumulative loss that occurs as a consequence of the high number of switching units with which any one light pulse must come in contact, since there is a signal loss associated with each encounter with a switching unit. Moreover, the signal losses (i.e. insertion losses) will vary significantly from path to path. For example, an optical signal that enters at input port 42 and exits at output port 52 will encounter one switching unit, while an optical signal entering at input port 48 and exiting at output port 58 will encounter seven switching units. What is needed is an optical add/drop switch having a minimal number of switching units, thereby allowing the construction of large switches with generally uniform, acceptably low loss.
A generally uniform reduction in the signal loss characteristics along transmission paths within an add/drop switch are achieved by configuring a matrix of switching arrangements such that propagation through the matrix requires contact with a maximum of two switching arrangements. In the preferred embodiment, the switching arrangements are fluid-containing trenches and the add/drop switch has an array of parallel first optical paths with input ports at first ends and drop ports at second ends. Each optical path is formed of generally aligned waveguides that intersect only one fluid-containing trench. The first optical paths form crossing patterns with second optical paths having add ports at first ends and output ports at second ends. The fluid in each trench is manipulated such that a light pulse is either reflected by or transmitted through the trench. Each trench is positioned and oriented at the intersection of first and second optical paths so that when the trench is in a reflective state, the input and output ports of the two optical paths are optically connected. However, when the trench is in a transmissive state, the add and output ports of the two optical paths are coupled, as are the input and drop ports.
In the preferred embodiment, each trench defines a switching arrangement that is responsive to the manipulation of index-matching fluid to change between the reflective state and the transmissive state. Typically, the fluid is a liquid having a refractive index that closely matches the refractive index of the optical core material of the waveguides. Consequently, when fluid resides at the interface of a trench with a waveguide, an optical signal propagating through the waveguide will enter the trench and propagate to the generally aligned waveguide that is on the opposite side of the trench. On the other hand, when there is an absence of liquid at the waveguide-to-trench interface, the trench is in a reflective state and any optical signal that reaches the interface will be reflected. In one embodiment, the number (N) of input ports is equal to the number for each of the output ports, add ports and drop ports. However, other embodiments are contemplated.
In a second embodiment, each switching arrangement is defined by a pair of trenches along each first optical path. The pair of trenches is operated as a two-state unit that is responsive to the manipulation of index-matching fluid to simultaneously change both trenches between reflective states and the transmissive states. As in the first embodiment, the fluid is typically a liquid having a refractive index that closely matches the refractive index of the optical core material of the waveguides. Consequently, when fluid resides at the interface of a trench with a waveguide, an optical signal propagating through the waveguide will enter the trench and propagate to the generally aligned waveguide that is on the opposite side of the trench. On the other hand, when there is an absence of liquid at the waveguide-to-trench interface, the trench is in a reflective state and any optical signal that reaches the interface will be reflected.
In this second embodiment, transmission by both of the trenches that reside along a particular first optical path causes optically aligned input and output ports to be coupled, while reflection by both trenches causes operatively associated input and drop ports to be coupled and causes operatively associated add and output ports to be coupled. Preferably, the number (N) of input ports is equal to the number of each of the output ports, add ports and drop ports. However, other embodiments are contemplated.
An advantage of the invention is that by limiting the number of switching arrangements, the device may be manufactured on a single substrate having a significantly reduced complexity. In the add/drop switch embodiment,xe2x80x9caddxe2x80x9d signals can be switched to either the associated output ports or the associated drop ports without rearranging existing connections. Another advantage of the preferred embodiment is that any light pulse transmitted from an input or add port will interact with only one switching arrangement, thereby providing a structure in which signal losses are low and are substantially equal along all of the possible optical paths.